FIG. 3 shows an example of a circuit configuration of a three-level inverter that converts power from direct current to alternating current. Direct current power supplies 1 and 2 are connected in series, wherein a positive electrode potential is taken to be P, a negative electrode potential N, and an intermediate point potential M. Generally, when the direct current power supplies are configured of an alternating current power supply system, an unshown rectifier and large capacity electrolytic capacitor can be configured of a 2-series connection, or the like.
A U-phase series circuit, wherein an IGBT T1 to which a diode D1 is connected in anti-parallel and an IGBT T2 to which a diode D2 is connected in anti-parallel are connected in series, a V-phase series circuit, wherein an IGBT T3 to which a diode D3 is connected in anti-parallel and an IGBT T4 to which a diode D4 is connected in anti-parallel are connected in series, and a W-phase series circuit, wherein an IGBT T5 to which a diode D5 is connected in anti-parallel and an IGBT T6 to which a diode D6 is connected in anti-parallel are connected in series, are connected in parallel to the series circuit of the direct current power supplies 1 and 2, thus configuring a three-phase bridge inverter circuit.
A U-phase bidirectional switch wherein reverse blocking IGBTs T7 and T8 are connected in anti-parallel is connected to a series connection point U of the U-phase series circuit and a connection point M of the direct current power supplies 1 and 2, a V-phase bidirectional switch wherein reverse blocking IGBTs T9 and T10 are connected in anti-parallel is connected to a series connection point V of the V-phase series circuit and the connection point M of the direct current power supplies 1 and 2, and a W-phase bidirectional switch wherein reverse blocking IGBTs T11 and T12 are connected in anti-parallel is connected to a series connection point W of the W-phase series circuit and the connection point M of the direct current power supplies 1 and 2. Also, the series connection points U, V, and W are connected to a motor 10, which is the load. Herein, the bidirectional switches can also be realized with a configuration wherein an IGBT that does not have reverse breakdown voltage and a diode are combined, as shown in FIG. 5(b).
As the P potential, N potential, and M potential can be output as the potential applied to the motor 10 by adopting this circuit configuration, a three-level output inverter is obtained. FIG. 4 shows an output voltage (Vout) waveform example. Three levels of voltage—direct current voltages 0, Ed1, and Ed1+Ed2—can be output. With this method, as low-order harmonic components decrease and it is possible to reduce switch element switching loss with respect to a two-level type inverter, it is possible to construct a highly efficient system.
FIG. 6 shows a one-phase system diagram including gate drive circuits that drive IGBTs and a control circuit that generates gate drive signals. 11a to 11d are connected between the gate and emitter of each IGBT in the gate drive circuits, and on-off control the IGBTs in accordance with gate drive signals 13a to 13d from the control circuit 12. Also, diodes 14a to 14d are connected with the object of detecting the potential of the collector portion of each IGBT, and detection of a power supply short circuit current (arm short circuit current) flowing due to failure of the corresponding IGBT or diode is carried out by detector circuits 15a to 15d in the gate drive circuits. At this time, failure detection signals 16a to 16d are output to the control circuit 12.
FIG. 7 shows other methods of detecting an arm short circuit current, wherein FIG. 7(a) is a method whereby the current value is detected by utilizing a sense IGBT 17 incorporated in an IGBT chip (in actuality, a resistor 18 is connected in series, and the voltage across the resistor 18 is detected). FIG. 7(b) shows a method whereby a shunt resistor 19 is connected in series with an IGBT, and the value of the voltage across the shunt resistor 19 is detected. Both methods are such that detection is carried out by detecting an excessive voltage generated across a resistor by an arm short circuit current.
For example, when the IGBT T7 or T8, which is an intermediate side element, fails as in FIG. 8(b) in the condition of FIG. 8(a) (a condition wherein the voltage Ed1 is supplied to the motor from the IGBTs T3 and T5 via the IGBT T7), an excessive power supply short circuit current 22 flows on the IGBT T1 being turned on, as in FIG. 8(c). In general, an IGBT is guaranteed for in the region of 10 μs not to be destroyed by a power supply short circuit current, because of which, provided that the power supply short circuit current is detected within 10 μs by the gate drive circuit of T1, which is a normal IGBT, and the gate is shut off, no secondary damage occurs.
That is, it is understood which arm semiconductor element has failed from the failure detection signal from the gate drive circuit in which a protection operation has been carried out.
A circuit example of the heretofore described three-level inverter is shown in PTL 1 , and a method of protecting an IGBT from a power supply short circuit current is shown in PTL 2.